1. Field of the Invention
Embodiments of the invention described herein pertain to the field of electric submersible pump (ESP) assemblies. More particularly, but not by way of limitation, one or more embodiments of the invention enable a torque transmitting coupling for an ESP equipment string.
2. Description of the Related Art
Electric submersible pumps (ESP) have been in use for nearly a century and over that time period little has changed mechanically from the historical design. Most improvements have been in the nature of better wire insulation, lubrication and bearing materials. Historical design, however, does not support the modern trend toward directional drilling, which causes bends in downhole wells. In directional drilling, the drill bit may be realigned from a traditional vertical direction to a horizontal direction to reach larger pockets of oil or other desirable resources. In doing so the bit travels in an arc with a fairly small radius. To reach resources in a horizontal direction the arc (degree of bend) of the hole will have to be large enough to allow the ESP equipment to pass through without any yielding of flanges, bolts, shaft couplings or housings. However, in some cases the exact depth and true distance to the bend required is unknown in advance. Unexpectedly, the ESP equipment may be required to bend more than the maximum safe bend of 10 degrees/100 feet, such as 25°/100 feet. A bend greater than 10°/100 feet can lead to permanent damage to the shaft couplings that transmit torque from each piece of equipment to the string, such as between two tandem motors, between the motor and seal section, between the seal section and gas separator and/or between the seal section and pump.
Another obstacle that occurs between two such shafts coupled together is a condition called “torque lock”. When two motors are coupled together in tandem, the upper (or downstream) motor may experience greater thermal expansion than the bottom (or upstream) motor at the coupling interface. The top shaft will expand up to ⅜ inch downwards. Since the coupling is transmitting torque between the shafts, the upper shaft must be allowed to move freely inside the coupling. If the torque is sufficient enough it will resist shaft movement and overload the lower motor's thrust bearing located just below the coupling. This will cause a complete failure of the entire ESP string. Torque lock is exacerbated if the equipment is in a bend, as would be the case if the motors were located in the arc transition inside the well bore.
As is apparent from the above, currently available shaft couplings are not engineered to support modern wells that contain bends or handle torque. Therefore, there is a need for a torque transmitting coupling for ESP equipment strings, to increase the bend tolerance of ESP assemblies beyond ten degrees per one-hundred feet and to better resist torque lock.